Volume expander



G. GADBOIS VOLUME EXPANDER Filed April 17,- ILS-)433` @@Y y Z Attorneys VVolume expansion.

Patented May 8, 1945 UNITED STATES PATENT OFFICE VOLUME EXPANDER Georges Gadbois, Sweetsburg, Quebec, Canada Application April 17, 1943, Serial No. 483,533 s claims. (ci. yria-r11) Thepresent invention relates to e electrical reproduction of sounds and, more particularly, to an improved circuit of volume expanders.

As` is well known, it is sometimes desirable to compress-'the range of intensities of transmitted,

forrecorded, sounds to render said sounds acceptable to the medium being used for transmission or recording. The reproduction of said compressed sounds, or signals, is thereafter `effectuated through means capable of restoring the vthis rectified component as a grid bias affecting the rst amp1ifying`stageor stages of the repro- But, experience has shown that operating on the grids of the first stages 'introduces disturbances 'in the system, such as overloading and 4parasitic impulses, which affect tion ofthe sounds or signals.

the nal rendi- *The present invention has been conceived to avoid-l the disadvantages noted above and, therefore,the objects may be stated as follows:

The main object of the invention resides in the provision of means for improving the emciency of volume eXpanders.

Another object is the provision of a circuit arrangement for volume expanders, whereby means other than the rst amplifying tube are used for A further object is the provision of a volume lexpander capable of high fidelity reproduction.

Stillanother object contemplates a volume ex- `'spandex'. ofthe character described which is highly flexible of application and easily controlled,

A still further object provides for an expander of the character described which is relatively inexpensive to set up and of reliable operation.

Other objects and advantages of the invention .will become apparent as the description progresses. p

, For purposes of illustration, and as examples, two possible embodiments of the invention are shown'in the annexed diagrammatical drawing,

-nents of the amplifier are conventional, well known and may comprise any of various circuits, the block system of illustration has been used. Each of said blocks is therefore lettered according to its general function and disposed in the circuit according to its relative position therein. In Figure l, the simplest arrangement is shown which consists of a rst amplifier, or pre-amplier as it is sometimes called, an intermediate ampliiier adapted to raise the pre-amplified signal, or sounds, to higher levels, and a power amplifier the output of which may be connected vto a transmission line or sound emitting means.

All said amplifiers are thermionically operated and are assumed to include their own sources of actuating potential although, in practice, they are all connected to a common source or power pack.

The expander 5 is connected to the input terminals, as shown in Figure 3, to apply the received signal across the grid 'I and cathode 8 of a tube `6 forv amplification purposes. But, the plate 9 of said tube is not connected to any source of constant potential and, accordingly, the said tube 6 should be inoperative normally.

In order to render tube 6. active, it is evident that a positive potential must be applied to the plate and, for that purpose, a portion of the output voltage of the power'amplier is bled therefrom, rectiiied through the rectifier l0 and applied to the plate 9 through a suitable plate load. Thus, under operative conditions, the potential applied to plate I0 shall be a variable potential of positive polarity, corresponding in all respects to the envelope of the pulsating currents present in the output of the power amplifier.

Consequently, the dynamic characteristics of the tube 6 are aiected in accordance with the variations occurring in the positive potential applied to its plate 9. As a result, the amplifying properties of said tube are also variable in direct proportion to the peakspresent in the power amplier output.

Therefore, the tube 6 amplies the signal of the input terminals in a variable manner, depending on the characteristics of the signal itself.` If the plate 9 of tube 6 is connected, through condenser Il, to the input of the intermediate amplifier, expanded impulses shall be superposed over the signal being amplified therein and will similarly affect the output of the power amplifier. The result is, consequently, an amplified signal expanded in a ratio strictly proportional to the characteristics of the signal itself. i

The object of the arrangement just described is to avoid using the first amplifying tube for expansion purposes, as is the common practice. The reasons for avoiding using said first tube are motivated by the fact that the permissible potential variations, in a grid circuit, are strictly limited, and complicated, by the grid bias voltage necessary for proper operation; the incoming signal, therefore, is generally all that the pre-amplifier can accept without distortion. When, over these signal impulses, an expanded image thereof is superposed, the combination is generally more than can be handled, kunless extreme care is used in feeding back only a very feeble, and ineffective, expanded image. Consequently, the use of a separate tube according to the invention, which tube is plate-controlled, is a complete solution to the problem set forth above, since even the ordinary so-called all purpose tubes are operable with plate voltages varying from a few volts to 250 or more.

Asa further refinement against grid distortion, a second embodiment of the invention is shown in Figure 2. wherein a separate volume expanding channel is shown. i This arrangement is preferred, because the expanded. signal impulses are not superposed vabove those amplified in the intermediate amplifier where, if `the 'design is not generous, distortion may stili occur should the expansion be allowed to overload same.

, Therefore, the expanded signal from expander is amplified directly through a separate arnplifying channel including intermediate and power amplifiers. The non-expanded signal impulses are preampliiied,as usual through the rst amplifier and, thereafter, raised to a level, suitable for proper expander operation, by means of a simple-voltage amplifier I2, which voltage amplifier is solely designed for raising the potential of the signal impulses. Due tothe fact that the current necessaryto operate the expander is slight,rsaid voltage amplifier may be constituted of simple elements and does not necessitate power tubes orV the like.

Thus, both the signalimpulses and the expanded impulses being handled through separate channels, the Vpossibilities of distortion are reduced to a minimum and, of course, the preamplifier is relieved ofthe task of amplifying both the signal and its expanded image.

From the foregoing, it should be apparent that the present invention represents an advance of the art: through the intermediary of simple means effective expansion of compressed signals y is easily achieved with gooddelity. AFurthermore, relatively more expansion is possible, with given apparatus, than is feasible using the conventional. system described previously and with the same permissible percentage of harmonics.

Finally, it should be evident that better oontrol of the qualityof reproduced sounds is possible, due to the separation of amplified signal and its expanded image into separate channels at the input.

Although, for purposes of simplification, the

drawing shows in Figure 3 a triode as. the expander tube 6, it is pointed out that in practice tetrodes are preferable since they are better adapted to prevent disturbances and other difiiculties due to the inter-electrode capacities.

It is to be understood that various changes in the arrangement of elements can be made, with plying said signal to the grid of a separate therm' ionic tube, rectifying a portion of the amplified-l signal impulses to obtain a direct current potential varying in accordance with the intensityof the signal impulses, using said potential as the sole plate voltage source for the thermionic tube so as to cause the dynamic characteristics thereof to change in conformity with' the piate voltage, and thereafter amplifying and reproducing the expanded signal amplified by the said tube.

2. -A volume expander of the` character described including a signal source and signal amplifying means, comprising a separate thermionic tube having its grid connected to the source, and means for applying to the plate of said tube a D. C. operating potential rectified from the signal amplifying meansand varying in accordance with the intensity of the signal component peaks, whereby the signal amplified by said tube is expanded in accordance with the characteristics of the signal itself. y

3. In a signal control system having a signal source and signal-amplifying means, signal expanding means having a separate thermionic tube connectedto the source and devoid of fixed plat-,e potential, means for applying to the plate of said tube a potential rectified from the output of the signal-amplifying means, and means for amplifying and reproducing the expanded signal appearing on the plate of said tube.

4. In an amplifier having input and output and the input of which is connected to a signal source, a separate expander tube connected to said source and including at least three elements of which one is a plate devoid of fixed plate voltage, a diode rectifier between said plate and the amplifier output, and means for amplifying and reproducing the signal appearing on the plate of the expander.

5. In a signal control method wherein signals from a source are amplified by a thermionic amp lier, causing rectification of a portion of said amplified signal to obtain a pulsating D. C; potential, using said potential as sole plate voltage for a separate thermionic amplifier, lapplying from the source the signals to said separate arnplier, and thereafter causing amplification and reproduction of the signals issuing from said separate amplifier.

6. A methodA of the characterl described, comprising thethermionic amplicationof signals jfrom a source in the usual manner, simultaneously applying the signals to aV separate indee GEoaGEs GADBoIs. 

